Page:The New International Encyclopædia 1st ed. v. 03.djvu/196

* BLEACHING. 166 BLEACHING-POWDER. called a hleaching-grccn. The fabric was sprin- kled vith water several times a day, and after several months of exposure it became wliite. Xo improvements of importance occxirred until the Eigliteenth Century, when Holland became the principal seat of the industry, and there the ma- terial was steeped in a solution of potash lye for several days and then in buttermilk for about a week, after which it was washed and bleached on the ground. The satisfactory result:, thus ob- tained led to the name 'Hollands' beinj; given to the excellent fabrics bleached in that way, while a quality of linen, nuich in demand, which was spread oil grass fields, was called 'lawn.' The next improvement was the use of dilute sulphuric acid in place of buttermilk, by which the dur.ation of the ijroccss, formerly about eight months, was re- duced to one-half that time. In 1785 the remark- able bleaching properties of chlorine were dis- covered by Berthollet. and its application to the bleaching of cloth soon followed. At first the chlorine was used in its gaseous state, and it was found that it destroyed" color by uniting chem- ically with the coloring principle, thus decom- posing the color; but as the chlorine also united with the hydrogen of the fibre it destroyed the fabric. Subsequently javelle-water. obtained by dissolving chlorine in dilute potassium hydrox- ide, was employed: but in 1790 the dry chloride of lime, or bleaching-powder, came into use. and is still largely employed, although for some pur- poses hydrogen peroxide is now preferred. CoTTON-ELEACHI^•G. Wlien yarn and thread are bleached, the process consists in boiling the ma- terial in a dilute solution of caustic alkali and washing; then steeping in a solution of bleaching- powder and washing, after which it is steeped in sulphuric acid and again washed, then heated in a soap solution with the additicm of a small quantity of bluing, after which it is passed through a dumping-machine consisting of wooden rollers, containing a solution of indigo, then washed and dried again. In the case of cotton cloth or calico the process is somewhat more complicated, but consists essentially in the treat- ment of the material in baths of lime. lye. bleach- ing-powder, and sulphuric acid, with thorough washing after each step. The most recent im- provement in this process has been the substitu- tion of caustic alkali .and resin soap for the lime- boil as the initial step. It should be mentioned that an electric bleaching process has been intro- duced, the principal feature of which is that the bleaching liquid, as hjiwchlorite, is obtained by passing an electric current through solutions of calcii'jn or magnesium chloride. % LiNE.N'-BLE.rinSG. The process employed for removing colors from linen is similar to that employed for cotton, although the natural im- purities are greater in the flax-fibre than in the cotton. These are rendered soluble by alkaline boilings, after which the coloring matters that still remain are oxidized and destroyed by baths of hypochlorites, or bleaching-powder. It is sometimes found desirable, especially when linen cloth is bleached, to introduce a step called grassinfi, which consists in ex))osing the material in a field to the action of air. light, and moistiire for several days. It is believed that by this method the full strength of the fibre is more completely retained. In linen-bleaching the treatment of the yarn varies slightly from the treatment of the cloth, the latter being somewhat more extended. Wool-Ble..ching. The special feature in which this difl'ers from the preceding methods is in the preliminary jjrocess, which is called scouring. Wool-fibre usually contains from 15 to 70 per cent, of foreign matter, consisting of dirt, and especially of fatty matters secreted by the ani- mal, called yolk. This is divided into two parts: (1) 'Wool perspiration,' which is soluble in water, and consists essentially of potassiinu com- pounds of oleic and stearic acids, thus constitut- ing an im])ortant source of potash; and (2) a compound insoluble in water and composed of cholestcrin in combination with fatty acids. The scouring process consists in dipping the loose wool on a frame into dilute alkaline solutions which form enuilsions with the yolk, or in treat- ing the wool to the successive action of fat sol- vents, such as carbon disulphide. and washing. When the wool-yarn is scoured the process is similar, although the machinery is sonu'what dif- ferent. ool cloth is scoured by passing the material as an endless band through the scouring liquid, and then through a pair of squeezing roll- ers. Even after scouring, wool cloth retains a. faint yellow tint, which is then removed by- bleaching. This is accomplished by the action of sulphurous acid, which is formed by burning sulphur in a closed chamber while the scoured and washed m;ilcrial, still wet, is suspended on poles, or the woolen material nuiy be immersed and moved about in a solution of sulphurous acid. After bleaching the materials are well washed, and any persistent yellow tint is neu- tralized by the addition of a preparation of bluing. Silk-Bleaching. Raw silk contains a gelati- nous substance called sericin. or silk glue, which, with other Calcareous and mineral matter, is re- moved by rinsing the silk in a warm bath of di- lute hydrochloric acid, and then washing. This process, called strii>]iiiig, is repeated until only a. small portion of the gmumy substance remains. The crude silk is then 'boiled olf for several hours in open boilers, after which the silk is well rinsed in a tepid solution of sodium carbonate, and finally in cold water. The crude material frequently loses as much as 30 per cent, by these operations. The bleaching process is similar to that described under Wool-Iileitrhing. and con- sists in exposing the wet scoured silk to the action of gaseous sulphurous acid. The use of hydrogen i)eroxide for bleaching silk is becoming rnore and more extensive. The method consists in stceiiing the silk in a dilute solution of hydro- gen peroxide, made slightly alkaline with am- monia, imtil it is sutUciently bleached. The bleaching of paper-stock, as cotton and linen rags or i)apcr. and the lileaching of straw, as well as the bleaching of animal proilucts, as bees- wax, feathers, hair, ivory, oils, and sponges, will be foiuid described in special articles under the names of these products. Consult: J. Gardner, Blriirhing. Dyeinii. and Calico-l'rinti)i;i (London, 18S4) : ;ind Modern lileaching and Fiiiishing, by a practical working bleacher (London, 1897). Works on dyeing usually contain special chap- ters (Ui bleaching. BLEACHING-POWDER, or Chloride of Lime. An important bleaching agent and disin- fectant. The exiict chemical nature of bleaching-